线程堆栈分析
本文为读书笔记，关于 Java 线程堆栈分析，在阅读并进行实验的基础上进行整理，如果有问题欢迎反馈
Java 虚拟机提供了线程转储（Thread dump）的后门，通过这个后门，可以将线程堆栈打印出来。这个后门就是通过向 Java 进程发送一个 QUIT 信号，Java 虚拟机收到该信号之后，将系统当前的 Java 线程调用堆栈打印出来。
kill -3 <java_pid> /jstack <java_pid>
1. Demo
public class MyTest {
Object obj1 = new Object();
Object obj2 = new Object();
public void fun1() {
synchronized(obj1) {
fun2();
}
}
public void fun2() {
synchronized(obj2) {
while(true) {
System.out.print("");
}
}
}
public static void main(String[] args) {
MyTest aa = new MyTest();
aa.fun1();
}
}
1.1 使用 kill -3 得到的结果
2017-12-26 10:59:58 Full thread dump Java HotSpot(TM) 64-Bit Server VM (24.76-b04 mixed mode): "Attach Listener" daemon prio=10 tid=0x00007fd9d4001000 nid=0x6993 runnable [0x0000000000000000] java.lang.Thread.State: RUNNABLE "Service Thread" daemon prio=10 tid=0x00007fda3c0a8800 nid=0x695f runnable [0x0000000000000000] java.lang.Thread.State: RUNNABLE "C2 CompilerThread1" daemon prio=10 tid=0x00007fda3c0a6000 nid=0x695e waiting on condition [0x0000000000000000] java.lang.Thread.State: RUNNABLE "C2 CompilerThread0" daemon prio=10 tid=0x00007fda3c0a3000 nid=0x695d waiting on condition [0x0000000000000000] java.lang.Thread.State: RUNNABLE "Signal Dispatcher" daemon prio=10 tid=0x00007fda3c0a0800 nid=0x695c waiting on condition [0x0000000000000000] java.lang.Thread.State: RUNNABLE "Finalizer" daemon prio=10 tid=0x00007fda3c078000 nid=0x695b in Object.wait() [0x00007fda32e06000] java.lang.Thread.State: WAITING (on object monitor) at java.lang.Object.wait(Native Method) - waiting on <0x0000000758c04858> (a java.lang.ref.ReferenceQueue$Lock) at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:135) - locked <0x0000000758c04858> (a java.lang.ref.ReferenceQueue$Lock) at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:151) at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:209) "Reference Handler" daemon prio=10 tid=0x00007fda3c076000 nid=0x695a in Object.wait() [0x00007fd9ec28e000] java.lang.Thread.State: WAITING (on object monitor) at java.lang.Object.wait(Native Method) - waiting on <0x0000000758c04470> (a java.lang.ref.Reference$Lock) at java.lang.Object.wait(Object.java:503) at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:133) - locked <0x0000000758c04470> (a java.lang.ref.Reference$Lock) "main" prio=10 tid=0x00007fda3c008800 nid=0x6950 runnable [0x00007fda4588a000] java.lang.Thread.State: RUNNABLE at MyTest.fun2(MyTest.java:12) - locked <0x0000000758c4a7f8> (a java.lang.Object) at MyTest.fun1(MyTest.java:6) - locked <0x0000000758c4a7e8> (a java.lang.Object) at MyTest.main(MyTest.java:18) "VM Thread" prio=10 tid=0x00007fda3c071800 nid=0x6959 runnable "GC task thread#0 (ParallelGC)" prio=10 tid=0x00007fda3c01e000 nid=0x6951 runnable "GC task thread#1 (ParallelGC)" prio=10 tid=0x00007fda3c020000 nid=0x6952 runnable "GC task thread#2 (ParallelGC)" prio=10 tid=0x00007fda3c022000 nid=0x6953 runnable "GC task thread#3 (ParallelGC)" prio=10 tid=0x00007fda3c024000 nid=0x6954 runnable "GC task thread#4 (ParallelGC)" prio=10 tid=0x00007fda3c026000 nid=0x6955 runnable "GC task thread#5 (ParallelGC)" prio=10 tid=0x00007fda3c027800 nid=0x6956 runnable "GC task thread#6 (ParallelGC)" prio=10 tid=0x00007fda3c029800 nid=0x6957 runnable "GC task thread#7 (ParallelGC)" prio=10 tid=0x00007fda3c02b800 nid=0x6958 runnable "VM Periodic Task Thread" prio=10 tid=0x00007fda3c0b3000 nid=0x6960 waiting on condition JNI global references: 123 Heap PSYoungGen total 150528K, used 7772K [0x0000000758c00000, 0x0000000763380000, 0x0000000800000000) eden space 129536K, 6% used [0x0000000758c00000,0x0000000759397200,0x0000000760a80000) from space 20992K, 0% used [0x0000000761f00000,0x0000000761f00000,0x0000000763380000) to space 20992K, 0% used [0x0000000760a80000,0x0000000760a80000,0x0000000761f00000) ParOldGen total 342528K, used 0K [0x000000060a400000, 0x000000061f280000, 0x0000000758c00000) object space 342528K, 0% used [0x000000060a400000,0x000000060a400000,0x000000061f280000) PSPermGen total 21504K, used 2429K [0x0000000605200000, 0x0000000606700000, 0x000000060a400000) object space 21504K, 11% used [0x0000000605200000,0x000000060545f738,0x0000000606700000)
1.2 jstack 的结果
2017-12-26 11:04:54 Full thread dump Java HotSpot(TM) 64-Bit Server VM (24.76-b04 mixed mode): "Attach Listener" daemon prio=10 tid=0x00007fd9d4001000 nid=0x6993 waiting on condition [0x0000000000000000] java.lang.Thread.State: RUNNABLE "Service Thread" daemon prio=10 tid=0x00007fda3c0a8800 nid=0x695f runnable [0x0000000000000000] java.lang.Thread.State: RUNNABLE "C2 CompilerThread1" daemon prio=10 tid=0x00007fda3c0a6000 nid=0x695e waiting on condition [0x0000000000000000] java.lang.Thread.State: RUNNABLE "C2 CompilerThread0" daemon prio=10 tid=0x00007fda3c0a3000 nid=0x695d waiting on condition [0x0000000000000000] java.lang.Thread.State: RUNNABLE "Signal Dispatcher" daemon prio=10 tid=0x00007fda3c0a0800 nid=0x695c runnable [0x0000000000000000] java.lang.Thread.State: RUNNABLE "Finalizer" daemon prio=10 tid=0x00007fda3c078000 nid=0x695b in Object.wait() [0x00007fda32e06000] java.lang.Thread.State: WAITING (on object monitor) at java.lang.Object.wait(Native Method) - waiting on <0x0000000758c04858> (a java.lang.ref.ReferenceQueue$Lock) at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:135) - locked <0x0000000758c04858> (a java.lang.ref.ReferenceQueue$Lock) at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:151) at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:209) "Reference Handler" daemon prio=10 tid=0x00007fda3c076000 nid=0x695a in Object.wait() [0x00007fd9ec28e000] java.lang.Thread.State: WAITING (on object monitor) at java.lang.Object.wait(Native Method) - waiting on <0x0000000758c04470> (a java.lang.ref.Reference$Lock) at java.lang.Object.wait(Object.java:503) at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:133) - locked <0x0000000758c04470> (a java.lang.ref.Reference$Lock) "main" prio=10 tid=0x00007fda3c008800 nid=0x6950 runnable [0x00007fda4588a000] java.lang.Thread.State: RUNNABLE at MyTest.fun2(MyTest.java:12) - locked <0x0000000758c4a7f8> (a java.lang.Object) at MyTest.fun1(MyTest.java:6) - locked <0x0000000758c4a7e8> (a java.lang.Object) at MyTest.main(MyTest.java:18) "VM Thread" prio=10 tid=0x00007fda3c071800 nid=0x6959 runnable "GC task thread#0 (ParallelGC)" prio=10 tid=0x00007fda3c01e000 nid=0x6951 runnable "GC task thread#1 (ParallelGC)" prio=10 tid=0x00007fda3c020000 nid=0x6952 runnable "GC task thread#2 (ParallelGC)" prio=10 tid=0x00007fda3c022000 nid=0x6953 runnable "GC task thread#3 (ParallelGC)" prio=10 tid=0x00007fda3c024000 nid=0x6954 runnable "GC task thread#4 (ParallelGC)" prio=10 tid=0x00007fda3c026000 nid=0x6955 runnable "GC task thread#5 (ParallelGC)" prio=10 tid=0x00007fda3c027800 nid=0x6956 runnable "GC task thread#6 (ParallelGC)" prio=10 tid=0x00007fda3c029800 nid=0x6957 runnable "GC task thread#7 (ParallelGC)" prio=10 tid=0x00007fda3c02b800 nid=0x6958 runnable "VM Periodic Task Thread" prio=10 tid=0x00007fda3c0b3000 nid=0x6960 waiting on condition JNI global references: 123
1.3 分析
用户线程为
"main" prio=10 tid=0x00007fda3c008800 nid=0x6950 runnable [0x00007fda4588a000] //main 表示线程名称，proi=1 表示线程优先级，tid=0x00007fda3c008800 表示线程 id，nid=0x6950 表示本地线程 id（这个查问题的时候会用到），runnable 表示线程状态，0x00007fda4588a000 表示内存地址 java.lang.Thread.State: RUNNABLE at MyTest.fun2(MyTest.java:12) //第一个MyTest 表示类名，fun2 表示方法名，第二个 MyTest.java 表示正在调用的函数所在的源代码文件，12 表示行号 - locked <0x0000000758c4a7f8> (a java.lang.Object) //locked 表示持有锁，后面的 0x0000000758c4a7f8 表示锁的 ID at MyTest.fun1(MyTest.java:6) - locked <0x0000000758c4a7e8> (a java.lang.Object) at MyTest.main(MyTest.java:18)
如何获得 Java 虚拟机对应的本地线程
1. jps //获取 Java 进程 ID 2. pstack <java_pid> 获得 Java 虚拟机的本地线程的堆栈
获得的本地线程栈如下所示
Thread 19 (Thread 0x7fda4588b700 (LWP 26960)): // Thread 19 表示线程名称，0x7fda4588b700 表示本地线程 id，LWP 26960 本地线程的另一种表示，LWP-light weight process（和上面的 nid 会一一对应，这里是十进制，上面的是十六进制） #0 0x00007fda401b2c47 in ?? () #1 0x00007fda401ab8fc in ?? () #2 0x0000000758c4a7f8 in ?? () #3 0x00007fda4588a8a0 in ?? () #4 0x0000000000000001 in ?? () #5 0x00007fda4014f058 in ?? () #6 0x00007fda4588a8f8 in ?? () #7 0x00007fda4014f058 in ?? () #8 0x0000000758c4a7d0 in ?? () #9 0x0000000000000001 in ?? () #10 0x0000000758c4a7e8 in ?? () #11 0x00007fda4588a8a8 in ?? () #12 0x000000060545d7b0 in ?? () #13 0x00007fda4588a918 in ?? () #14 0x000000060545dc80 in ?? () #15 0x0000000000000000 in ?? () Thread 18 (Thread 0x7fda40148700 (LWP 26961)): #0 0x00007fda4546b68c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0 #1 0x00007fda44652543 in os::PlatformEvent::park() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #2 0x00007fda4461987f in Monitor::IWait(Thread*, long) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #3 0x00007fda4461a00e in Monitor::wait(bool, long, bool) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #4 0x00007fda4438dad3 in GCTaskManager::get_task(unsigned int) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #5 0x00007fda4438f198 in GCTaskThread::run() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #6 0x00007fda44658ca8 in java_start(Thread*) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #7 0x00007fda45467aa1 in start_thread () from /lib64/libpthread.so.0 #8 0x00007fda44d99aad in clone () from /lib64/libc.so.6 Thread 17 (Thread 0x7fda3a5b3700 (LWP 26962)): #0 0x00007fda4546b68c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0 #1 0x00007fda44652543 in os::PlatformEvent::park() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #2 0x00007fda4461987f in Monitor::IWait(Thread*, long) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #3 0x00007fda4461a00e in Monitor::wait(bool, long, bool) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #4 0x00007fda4438dad3 in GCTaskManager::get_task(unsigned int) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #5 0x00007fda4438f198 in GCTaskThread::run() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #6 0x00007fda44658ca8 in java_start(Thread*) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #7 0x00007fda45467aa1 in start_thread () from /lib64/libpthread.so.0 #8 0x00007fda44d99aad in clone () from /lib64/libc.so.6 Thread 16 (Thread 0x7fda3a4b2700 (LWP 26963)): #0 0x00007fda4546b68c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0 #1 0x00007fda44652543 in os::PlatformEvent::park() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #2 0x00007fda4461987f in Monitor::IWait(Thread*, long) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #3 0x00007fda4461a00e in Monitor::wait(bool, long, bool) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #4 0x00007fda4438dad3 in GCTaskManager::get_task(unsigned int) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #5 0x00007fda4438f198 in GCTaskThread::run() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #6 0x00007fda44658ca8 in java_start(Thread*) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #7 0x00007fda45467aa1 in start_thread () from /lib64/libpthread.so.0 #8 0x00007fda44d99aad in clone () from /lib64/libc.so.6 Thread 15 (Thread 0x7fda3a3b1700 (LWP 26964)): #0 0x00007fda4546b68c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0 #1 0x00007fda44652543 in os::PlatformEvent::park() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #2 0x00007fda4461987f in Monitor::IWait(Thread*, long) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #3 0x00007fda4461a00e in Monitor::wait(bool, long, bool) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #4 0x00007fda4438dad3 in GCTaskManager::get_task(unsigned int) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #5 0x00007fda4438f198 in GCTaskThread::run() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #6 0x00007fda44658ca8 in java_start(Thread*) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #7 0x00007fda45467aa1 in start_thread () from /lib64/libpthread.so.0 #8 0x00007fda44d99aad in clone () from /lib64/libc.so.6 Thread 14 (Thread 0x7fda3a2b0700 (LWP 26965)): #0 0x00007fda4546b68c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0 #1 0x00007fda44652543 in os::PlatformEvent::park() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #2 0x00007fda4461987f in Monitor::IWait(Thread*, long) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #3 0x00007fda4461a00e in Monitor::wait(bool, long, bool) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #4 0x00007fda4438dad3 in GCTaskManager::get_task(unsigned int) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #5 0x00007fda4438f198 in GCTaskThread::run() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #6 0x00007fda44658ca8 in java_start(Thread*) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #7 0x00007fda45467aa1 in start_thread () from /lib64/libpthread.so.0 #8 0x00007fda44d99aad in clone () from /lib64/libc.so.6 Thread 13 (Thread 0x7fda3a1af700 (LWP 26966)): #0 0x00007fda4546b68c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0 #1 0x00007fda44652543 in os::PlatformEvent::park() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #2 0x00007fda4461987f in Monitor::IWait(Thread*, long) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #3 0x00007fda4461a00e in Monitor::wait(bool, long, bool) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #4 0x00007fda4438dad3 in GCTaskManager::get_task(unsigned int) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #5 0x00007fda4438f198 in GCTaskThread::run() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #6 0x00007fda44658ca8 in java_start(Thread*) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #7 0x00007fda45467aa1 in start_thread () from /lib64/libpthread.so.0 #8 0x00007fda44d99aad in clone () from /lib64/libc.so.6 Thread 12 (Thread 0x7fda3a0ae700 (LWP 26967)): #0 0x00007fda4546b68c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0 #1 0x00007fda44652543 in os::PlatformEvent::park() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #2 0x00007fda4461987f in Monitor::IWait(Thread*, long) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #3 0x00007fda4461a00e in Monitor::wait(bool, long, bool) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #4 0x00007fda4438dad3 in GCTaskManager::get_task(unsigned int) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #5 0x00007fda4438f198 in GCTaskThread::run() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #6 0x00007fda44658ca8 in java_start(Thread*) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #7 0x00007fda45467aa1 in start_thread () from /lib64/libpthread.so.0 #8 0x00007fda44d99aad in clone () from /lib64/libc.so.6 Thread 11 (Thread 0x7fda39fad700 (LWP 26968)): #0 0x00007fda4546b68c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0 #1 0x00007fda44652543 in os::PlatformEvent::park() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #2 0x00007fda4461987f in Monitor::IWait(Thread*, long) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #3 0x00007fda4461a00e in Monitor::wait(bool, long, bool) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #4 0x00007fda4438dad3 in GCTaskManager::get_task(unsigned int) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #5 0x00007fda4438f198 in GCTaskThread::run() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #6 0x00007fda44658ca8 in java_start(Thread*) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #7 0x00007fda45467aa1 in start_thread () from /lib64/libpthread.so.0 #8 0x00007fda44d99aad in clone () from /lib64/libc.so.6 Thread 10 (Thread 0x7fda32f08700 (LWP 26969)): #0 0x00007fda4546ba5e in pthread_cond_timedwait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0 #1 0x00007fda44657c0f in os::PlatformEvent::park(long) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #2 0x00007fda44619b2e in Monitor::IWait(Thread*, long) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #3 0x00007fda4461a00e in Monitor::wait(bool, long, bool) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #4 0x00007fda447e1899 in VMThread::loop() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #5 0x00007fda447e1ba0 in VMThread::run() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #6 0x00007fda44658ca8 in java_start(Thread*) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #7 0x00007fda45467aa1 in start_thread () from /lib64/libpthread.so.0 #8 0x00007fda44d99aad in clone () from /lib64/libc.so.6 Thread 9 (Thread 0x7fd9ec28f700 (LWP 26970)): #0 0x00007fda4546b68c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0 #1 0x00007fda44652543 in os::PlatformEvent::park() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #2 0x00007fda44646ddd in ObjectMonitor::wait(long, bool, Thread*) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #3 0x00007fda444b6298 in JVM_MonitorWait () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #4 0x00007fda4015bd98 in ?? () #5 0x00007fda3c0768d8 in ?? () #6 0x00007fd900000000 in ?? () #7 0x00007fda3c076000 in ?? () #8 0x00007fd9ec28e5d8 in ?? () #9 0x0000000605202798 in ?? () #10 0x00007fd9ec28e648 in ?? () #11 0x00000006052a9e68 in ?? () #12 0x0000000000000000 in ?? () Thread 8 (Thread 0x7fda32e07700 (LWP 26971)): #0 0x00007fda4546b68c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0 #1 0x00007fda44652543 in os::PlatformEvent::park() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #2 0x00007fda44646ddd in ObjectMonitor::wait(long, bool, Thread*) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #3 0x00007fda444b6298 in JVM_MonitorWait () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #4 0x00007fda4015bd98 in ?? () #5 0x00007fda3c078a08 in ?? () #6 0x00007fda00000000 in ?? () #7 0x00007fda3c078000 in ?? () #8 0x00007fda32e065e8 in ?? () #9 0x0000000000000000 in ?? () Thread 7 (Thread 0x7fda32d06700 (LWP 26972)): #0 0x00007fda4546da00 in sem_wait () from /lib64/libpthread.so.0 #1 0x00007fda446578ea in check_pending_signals(bool) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #2 0x00007fda44650b35 in signal_thread_entry(JavaThread*, Thread*) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #3 0x00007fda447930ff in JavaThread::thread_main_inner() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #4 0x00007fda44793205 in JavaThread::run() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #5 0x00007fda44658ca8 in java_start(Thread*) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #6 0x00007fda45467aa1 in start_thread () from /lib64/libpthread.so.0 #7 0x00007fda44d99aad in clone () from /lib64/libc.so.6 Thread 6 (Thread 0x7fda32c05700 (LWP 26973)): #0 0x00007fda4546b68c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0 #1 0x00007fda44652543 in os::PlatformEvent::park() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #2 0x00007fda4461987f in Monitor::IWait(Thread*, long) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #3 0x00007fda4461a086 in Monitor::wait(bool, long, bool) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #4 0x00007fda44282648 in CompileQueue::get() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #5 0x00007fda4428674a in CompileBroker::compiler_thread_loop() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #6 0x00007fda447930ff in JavaThread::thread_main_inner() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #7 0x00007fda44793205 in JavaThread::run() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #8 0x00007fda44658ca8 in java_start(Thread*) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #9 0x00007fda45467aa1 in start_thread () from /lib64/libpthread.so.0 #10 0x00007fda44d99aad in clone () from /lib64/libc.so.6 Thread 5 (Thread 0x7fda32b04700 (LWP 26974)): #0 0x00007fda4546b68c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0 #1 0x00007fda44652543 in os::PlatformEvent::park() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #2 0x00007fda4461987f in Monitor::IWait(Thread*, long) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #3 0x00007fda4461a086 in Monitor::wait(bool, long, bool) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #4 0x00007fda44282648 in CompileQueue::get() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #5 0x00007fda4428674a in CompileBroker::compiler_thread_loop() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #6 0x00007fda447930ff in JavaThread::thread_main_inner() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #7 0x00007fda44793205 in JavaThread::run() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #8 0x00007fda44658ca8 in java_start(Thread*) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #9 0x00007fda45467aa1 in start_thread () from /lib64/libpthread.so.0 #10 0x00007fda44d99aad in clone () from /lib64/libc.so.6 Thread 4 (Thread 0x7fda32a03700 (LWP 26975)): #0 0x00007fda4546b68c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0 #1 0x00007fda44652543 in os::PlatformEvent::park() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #2 0x00007fda4461987f in Monitor::IWait(Thread*, long) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #3 0x00007fda4461a00e in Monitor::wait(bool, long, bool) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #4 0x00007fda446e95a8 in ServiceThread::service_thread_entry(JavaThread*, Thread*) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #5 0x00007fda447930ff in JavaThread::thread_main_inner() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #6 0x00007fda44793205 in JavaThread::run() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #7 0x00007fda44658ca8 in java_start(Thread*) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #8 0x00007fda45467aa1 in start_thread () from /lib64/libpthread.so.0 #9 0x00007fda44d99aad in clone () from /lib64/libc.so.6 Thread 3 (Thread 0x7fda32902700 (LWP 26976)): #0 0x00007fda4546ba5e in pthread_cond_timedwait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0 #1 0x00007fda44657c0f in os::PlatformEvent::park(long) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #2 0x00007fda44619b2e in Monitor::IWait(Thread*, long) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #3 0x00007fda4461a00e in Monitor::wait(bool, long, bool) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #4 0x00007fda4478f121 in WatcherThread::sleep() const () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #5 0x00007fda4478f63e in WatcherThread::run() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #6 0x00007fda44658ca8 in java_start(Thread*) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #7 0x00007fda45467aa1 in start_thread () from /lib64/libpthread.so.0 #8 0x00007fda44d99aad in clone () from /lib64/libc.so.6 Thread 2 (Thread 0x7fda32801700 (LWP 27027)): #0 0x00007fda4546eb2d in accept () from /lib64/libpthread.so.0 #1 0x00007fda44136af1 in LinuxAttachListener::dequeue() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #2 0x00007fda44136c8b in AttachListener::dequeue() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #3 0x00007fda441357da in attach_listener_thread_entry(JavaThread*, Thread*) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #4 0x00007fda447930ff in JavaThread::thread_main_inner() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #5 0x00007fda44793205 in JavaThread::run() () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #6 0x00007fda44658ca8 in java_start(Thread*) () from /usr/local/jdk1.7.0_76/jre/lib/amd64/server/libjvm.so #7 0x00007fda45467aa1 in start_thread () from /lib64/libpthread.so.0 #8 0x00007fda44d99aad in clone () from /lib64/libc.so.6 Thread 1 (Thread 0x7fda4588d700 (LWP 26959)): #0 0x00007fda454682fd in pthread_join () from /lib64/libpthread.so.0 #1 0x00007fda45256505 in ContinueInNewThread0 () from /usr/local/jdk1.7.0_76/bin/../lib/amd64/jli/libjli.so #2 0x00007fda4524b58a in ContinueInNewThread () from /usr/local/jdk1.7.0_76/bin/../lib/amd64/jli/libjli.so #3 0x00007fda4524e0e0 in JLI_Launch () from /usr/local/jdk1.7.0_76/bin/../lib/amd64/jli/libjli.so #4 0x0000000000400686 in main ()
线程状态中的 runnable 从虚拟机的角度来看，表示这个线程正在运行。但是出于 runnable 的线程不一定真正消耗 CPU。出于 runnable 的线程只能说明该线程没有阻塞在 java 的 wait 或者 sleep 方法上，同时也没有等待在锁上面。但是如果该线程调用了本地方法（比如 java.net.SocketInputStream.socketRead0) ，而本地方法出于等待状态，这个时候虚拟机是不知道本地代码中发生了什么，此时尽管当前线程实际上也是阻塞状态，但实际上显示出来的还是 runnable 状态，这种情况下是不消耗 CPU 的。如下线程堆栈所示
"Thread-243" prio=1 tid=0xa58f2048 nid=0x7ac2 runnable [0xaeedb000..0xaeedc480]
at java.net.SocketInputStream.socketRead0(Native Method)
at java.net.SocketInputStream.read(SocketInputStream.java:129)
at oracle.net.ns.Packet.receive(Unknown Source)
at oracle.net.ns.DataPacket.receive(Unknown Source)
at oracle.net.ns.NetInputStream.getNextPacket(Unknown Source)
at oracle.net.ns.NetInputStream.read(Unknown Source)
at oracle.jdbc.driver.T4CMAREngine.getNBytes(T4CMAREngine.java:1520)
at oracle.jdbc.driver.T4CMAREngine.unmarshalNBytes()
at oracle.jdbc.driver.T4CLongRawAccessor.readStreamFromWire()
at oracle.jdbc.driver.T4CLongRawAccessor.readStream()
at oracle.jdbc.driver.T4CInputStream.getBytes(T4CInputStream.java:70)
- locked <0x934f4258> (a oracle.jdbc.driver.T4CInputStream)
- locked <0x6b0dd600> (a oracle.jdbc.driver.T4CConnection)
at oracle.jdbc.driver.OracleInputStream.needBytes()
... ...
at org.hibernate.loader.Loader.list(Loader.java:1577)
at org.hibernate.loader.hql.QueryLoader.list()
at com.wes.timer.TimerTaskImpl.execute(TimerTaskImpl.java:627)
- locked <0x80df8ce8> (a com.wes.timer.TimerTaskImpl)
at com.wes.threadpool.RunnableWrapper.run(RunnableWrapper.java:209)
at com.wes.threadpool.PooledExecutorEx$Worker.run()
at java.lang.Thread.run(Thread.java:595)
那么 “.
除接口外，初始化一个类之前必须保证其直接父类已被初始化，并且该初始化过程是由 JVM 保证线程安全的。并非所有的类都拥有一个
- 该类既没有声明任何类变量，也没有静态初始化语句
- 该类声明了类变量，但没有明确使用类变量初始化语句或静态初始化语句初始化
- 该类仅包含静态 final 变量的类变量初始化语句，并且类变量初始化语句是编译时常量表达式
2 锁的解读
多线程中，wait() 和 sleep() 的区别。wait() 和 sleep() 有一个共同点，就是二者都会把当前的线程阻塞住，我们称之为睡眠或者等待。但二者实际上是完全不同的两个函数，有着本质的区别：
- 当线程执行到 wait() 方法上，当前线程会释放监视锁，此时其他线程可以占有该锁，一旦 wait() 方法执行完成，当前线程又继续持有该锁，直到执行完该锁的作用域。
wait() 方法退出的原因：- 达到了等待时间，自动退出。如 wati(5000), 5秒后 wait 方法退出
- 其他的线程调用了该锁的 notify() 方法。当如果多个线程在等待同一个锁，只有一个线程会被通知到
- sleep() 与锁操作无关，如果该方法恰好在一个锁的保护范围之内，当前线程及时在执行 sleep() 的时候，仍然继续保持监视锁。该方法实际上仅仅完成等待或者睡眠的语义
与锁相关的三个重要信息如下：
- 当一个线程占有一个锁的时候，线程堆栈中会打印 – locked<0x22bffb60>
- 当一个线程正在等待其他线程释放该锁，线程堆栈中会打印 – waiting to lock<0x22bffb60>
- 当一个线程占有一个锁，但有执行到该锁的 wait() 上，线程堆栈中首先打印 locked, 然后又会打印 – waiting on<0x22c03c60>
3 线程状态的解读
3.1 RUNNABLE
RUNNABLE 从虚拟机的角度看，线程处于正在运行的状态，正在运行不一定消耗 CPU，比如下面的线程栈，在等待 IO
Thread-39" daemon prio=1 tid=0x08646590 nid=0x666d runnable [5beb7000..5beb88b8]
java.lang.Thread.State: RUNNABLE
at java.net.SocketInputStream.socketRead0(Native Method)
at java.net.SocketInputStream.read(SocketInputStream.java:129)
at java.io.BufferedInputStream.fill(BufferedInputStream.java:183)
at java.io.BufferedInputStream.read(BufferedInputStream.java:201)
- locked <0x47bfb940> (a java.io.BufferedInputStream)
at org.postgresql.PG_Stream.ReceiveChar(PG_Stream.java:141)
at org.postgresql.core.QueryExecutor.execute(QueryExecutor.java:68)
- locked <0x47bfb758> (a org.postgresql.PG_Stream)
at org.postgresql.Connection.ExecSQL(Connection.java:398)
下面的线程栈则消耗 CPU
"Thread-444" prio=1 tid=0xa4853568 nid=0x7ade runnable [0xafcf7000..0xafcf8680]
java.lang.Thread.State: RUNNABLE
//实实在在对应 CPU 指令
at org.apache.commons.collections.ReferenceMap.getEntry(Unknown Source)
at org.apache.commons.collections.ReferenceMap.get(Unknown Source)
at org.hibernate.util.SoftLimitMRUCache.get(SoftLimitMRUCache.java:51)
at org.hibernate.engine.query.QueryPlanCache.getNativeSQLQueryPlan()
at org.hibernate.impl.AbstractSessionImpl.getNativeSQLQueryPlan()
at org.hibernate.impl.AbstractSessionImpl.list()
at org.hibernate.impl.SQLQueryImpl.list(SQLQueryImpl.java:164)
at com.mogoko.struts.logic.user.LeaveMesManager.getCommentByShopId()
at com.mogoko.struts.action.shop.ShopIndexBaseInfoAction.execute()
......
下面的代码正在进行 JNI 本地方法调用，具体是否消耗 CPU，要看 TcpRecvExt 的实现，如果 TcpRecvExt 是纯运算代码，那么实实在在的消耗 CPU，如果 TcpRecvExt() 中存在挂起的代码，那么该线程尽管显示 RUNNABLE，实际也不消耗 CPU 的
"ClientReceiveThread" daemon prio=1 tid=0x99dbacf8 nid=0x7988 runnable [...]
java.lang.Thread.State: RUNNABLE
at com.pangu.network.icdcomm.htcpapijni.TcpRecvExt(Native Method)
at com.pangu.network.icdcomm.IcdComm.receive(IcdComm.java:60)
at com.msp.client.MspFactory$ClientReceiveThread.task(MspFactory.java:333)
at com.msp.system.TaskThread.run(TaskThread.java:94)
3.2 TIMED_WAITING(on object monitor)
TIMED_WAITING(on object monitor) 表示当前线程被挂起一段时间，说明该线程正在执行 object.wait(int time) 方法，不消耗 CPU
"JMX server" daemon prio=6 tid=0x0ad2c800 nid=0xdec in Object.wait() [...] java.lang.Thread.State: TIMED_WAITING (on object monitor) at java.lang.Object.wait(Native Method) - waiting on <0x03129da0> (a [I) at com.sun.jmx.remote.internal.ServerComm$Timeout.run(ServerComm.java:150) - locked <0x03129da0> (a [I) at java.lang.Thread.run(Thread.java:620)
3.3 TIME_WAITING(sleeping)
TIME_WAITING(sleeping) 表示当前线程被挂起一段时间，正在执行 Thread.sleep(int time) 方法，不消耗 CPU
"Comm thread" daemon prio=10 tid=0x00002aaad4107400 nid=0x649f waiting on condition
[0x000000004133b000..0x000000004133ba00]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep(Native Method)
at org.apache.hadoop.mapred.Task$1.run(Task.java:282)
at java.lang.Thread.run(Thread.java:619)
3.4 TIME_WAITING(parking)
TIME_WAITING(parking) 表示线程被挂起，正在执行 Thread.sleep(int time) 方法，不消耗 CPU
"RMI TCP" daemon prio=6 tid=0x0ae3b800 nid=0x958 waiting on condition [0x17eff000..0x17effa94]
java.lang.Thread.State: TIMED_WAITING (parking)
at sun.misc.Unsafe.park(Native Method)
- parking to wait for <0x02f49f58> (a java.util.concurrent.SynchronousQueue$TransferStack)
at java.util.concurrent.locks.LockSupport.parkNanos(LockSupport.java:179)
at java.util.concurrent.SynchronousQueue$TransferStack.awaitFulfill(SynchronousQueue.java:424)
at java.util.concurrent.SynchronousQueue$TransferStack.transfer(SynchronousQueue.java:323)
at java.util.concurrent.SynchronousQueue.poll(SynchronousQueue.java:871)
at java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:495)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:693)
at java.lang.Thread.run(Thread.java:620)
3.5 WAITING(on object monitor)
WAITING(on object monitor) 表示当前线程被挂起，正在执行 obj.wait() 只能通过 notify() 唤醒，不消耗 CPU
"IPC Client" daemon prio=10 tid=0x00002aaad4129800 nid=0x649d in Object.wait() [0x039000..0x039d00]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x00002aaab3acad18>; (aorg.apache.hadoop.ipc.Client$Connection)
at java.lang.Object.wait(Object.java:485)
at org.apache.hadoop.ipc.Client$Connection.waitForWork(Client.java:234)
- locked <0x00002aaab3acad18> (aorg.apache.hadoop.ipc.Client$Connection)
at org.apache.hadoop.ipc.Client$Connection.run(Client.java:273)
3.6 小结
处于 TIME_WAITING、WAITING 状态的线程不消耗 CPU，处于 RUNNABLE 状态的线程要结合线程代码性质判断，是否消耗 CPU
- 如果是纯 Java 运算代码，消耗 CPU
- 如果是网络 IO，很少消耗 CPU
- 如果是本地代码，结合本地代码性质进行判断（通过 pstack/gstack 获取本地线程栈），如果是纯运算代码，消耗 CPU，如果被挂起，不消耗 CPU，如果是 IO，则不怎么消耗 CPU
4 接触线程堆栈进行问题分析
线程堆栈翻译系统当前时间正在执行什么代码。根据这些信息就可以知道系统当前到底在做什么。看堆栈一般从三个视角来分析：堆栈的局部信息、一次堆栈的统计信息（全局信息）、多个堆栈的对比信息
- 视角一 从一次的堆栈信息中，我们能直接获取以下直接的信息：
a. 当前每一个线程的调用层次关系（调用上下文），即每个线程当前正在调用哪些函数
b. 当前每个线程当前的状态：持有了哪些锁？在等待哪些锁？ - 视角二 从一次的堆栈信息中，我们还可以获取下面的统计方面信息：
a. 当前锁的争用情况：i. 是不是很多线程在等待同一个锁，如果很多线程等待同一个锁，说明这个系统已经出现了性能瓶颈，并导致了锁竞争。还可能是某个线程长时间持有一个锁不释放（比如这个线程正陷入了死循环的代码或者正在请求一个资源，很长时间得不到唤醒） ii. 是否有死锁，哪些线程形成了死锁
b. 当前大多数线程在干什么，即正在执行什么代码
c. 当前线程总数量 - 视角三 从多次（即前后打印多次堆栈进行对比）的堆栈信息中，我们还可以获得下面的统计对比方面的信息：
a. 一个线程是否在长期执行。如果每次打印的堆栈，某一个线程一直处于同样的调用上下文中，那么说明这个线程一直在执行这段代码，此时就要根据代码逻辑检查，这种长期执行是否合理？
b. 某个线程是否存在长期获取不到锁的情况？线程是不是永远得不到唤醒？如果每次打印的堆栈，某一个线程一直在等待一个锁，那么就需要检查占有这个锁的线程为什么不释放锁？
通过多个视角进行观察，线程堆栈在定位如下类型的问题上非常有帮助：
- 线程死锁分析（视角一）
- Java 代码导致的 CPU 过高分析（视角三）
- 死循环分析（视角三）
- 资源不足分析（视角二）
- 性能瓶颈分析（视角二和视角三）
4.1 线程死锁分析
死锁的原因：当两个或多个线程正在等待被对方占有的锁，死锁就会发生。死锁会导致两个线程无法继续运行，被永远挂起。下图中在时间点0，线程 0 占有了 lock0，线程1占有了 lock1，然后时间点 2 线程0 企图获取 lock 1，线程 1 企图获取 lock 0.由于这两个线程互相要等待被对方占有的锁，自己才能继续，因此造成死锁，二者永远没有机会继续运行下去。
一个简单的例子
package MyPackage;
public class Main {
public static void main(String[] args) {
Object lockobj1 = new Object();
Object lockobj2 = new Object();
TestThread1 thread1 = new TestThread1(lockobj1,lockobj2);
thread1.start();
TestThread2 thread2 = new TestThread2(lockobj1,lockobj2);
thread2.start();
}
}
package MyPackage;
public class TestThread1 extends Thread{
Object lock1 = null;
Object lock2 = null;
public TestThread1(Object lock1_,Object lock2_)
{
lock1 = lock1_;
lock2 = lock2_;
this.setName(this.getClass().getName());
}
public void run()
{
fun();
}
public void fun(){
synchronized(lock1){
try{
Thread.sleep(2);
}
catch(Exception e){
e.printStackTrace();
}
synchronized(lock2){
}
}
}
}
package MyPackage;
public class TestThread2 extends Thread{
Object lock1 = null;
Object lock2 = null;
public TestThread2(Object lock1_,Object lock2_)
{
lock1 = lock1_;
lock2 = lock2_;
this.setName(this.getClass().getName());
}
public void run()
{
fun();
}
public void fun(){
synchronized(lock2){
try{
Thread.sleep(2);
}
catch(Exception e){
e.printStackTrace();
}
synchronized(lock1){
}
}
}
}
jstack 得到结果如下（截取后的堆栈信息）
Found one Java-level deadlock: ============================= "MyPackage.TestThread2": waiting to lock monitor 0x00007fa1b00062c8 (object 0x0000000758c4aa78, a java.lang.Object), which is held by "MyPackage.TestThread1" "MyPackage.TestThread1": waiting to lock monitor 0x00007fa1b0004ed8 (object 0x0000000758c4aa88, a java.lang.Object), which is held by "MyPackage.TestThread2" Java stack information for the threads listed above: =================================================== "MyPackage.TestThread2": at MyPackage.TestThread2.fun(TestThread2.java:24) - waiting to lock <0x0000000758c4aa78> (a java.lang.Object) - locked <0x0000000758c4aa88> (a java.lang.Object) at MyPackage.TestThread2.run(TestThread2.java:13) "MyPackage.TestThread1": at MyPackage.TestThread1.fun(TestThread1.java:24) - waiting to lock <0x0000000758c4aa88> (a java.lang.Object) - locked <0x0000000758c4aa78> (a java.lang.Object) at MyPackage.TestThread1.run(TestThread1.java:13) Found 1 deadlock.
要避免死锁的问题，唯一的办法就是修改代码。一个可靠的并发系统可以说是设计出来的，而不是通过改 Bug 改出来的，这一点与其他类型的 Bug 有很大的不同。另外，死锁的两个或多个线程是不消耗 CPU 的，有的人认为 CPU 100% 的使用率是线程死锁造成的，这个说法是不对的。无限循环（死循环），并且循环中代码都是 CPU 密集型，才有可能导致 CPU 的 100% 使用率，像 socket 或者数据库等 IO 操作是不怎么消耗 CPU 的。
4.2 Java 代码死循环等导致的 CPU 过高分析
当系统负载大的时候，CPU 的使用率会较高，但是不正确的代码也会导致 CPU 过高，比如死循环。当发生 CPU 过高的问题，我们需要能够分析 CPU 高的真正原因。通过多次打印堆栈，前后堆栈对比找到一直在运行的线程，这些线程都是可疑的线程，具体的步骤如下：
- 通过 jstack 等获取第一次的堆栈信息
- 等待一段时间，再获取第二次堆栈信息
- 预处理两次堆栈信息，首先去掉处于 sleeping 或者 waiting 状态的线程，因为这种线程是不消耗 CPU 的
- 比较第一次堆栈和第二次堆栈预处理后的线程，找出这段时间一直活跃的线程，如果两次堆栈中同一个线程处于同样的调用上下文，那么就应该列为重点怀疑对象。结合代码进行检查
如果通过堆栈定位，没有发现热点代码段，那么 CPU 过高可能是不恰当的内存设置导致的频繁 GC，而从导致 CPU 过高
第一次 jstack 的输出如下：
"Thread-444" prio=1 tid=0xa4853568 nid=0x7ade runnable [0xafcf7000..0xafcf8680]
at org.apache.commons.collections.ReferenceMap.getEntry(Unknown Source)
at org.apache.commons.collections.ReferenceMap.get(Unknown Source)
at org.hibernate.util.SoftLimitMRUCache.get(SoftLimitMRUCache.java:51)
at org.hibernate.engine.query.QueryPlanCache.getNativeSQLQueryPlan()
at org.hibernate.impl.AbstractSessionImpl.getNativeSQLQueryPlan()
at org.hibernate.impl.AbstractSessionImpl.list()
at org.hibernate.impl.SQLQueryImpl.list(SQLQueryImpl.java:164)
at com.mogoko.struts.logic.user.LeaveMesManager.getCommentByShopId()
at com.mogoko.struts.action.shop.ShopIndexBaseInfoAction.execute()
......
第二次 jstack 的输出如下：
"Thread-444" prio=1 tid=0xa4853568 nid=0x7ade runnable [0xafcf7000..0xafcf8680]
at org.apache.commons.collections.ReferenceMap.getEntry(Unknown Source)
at org.apache.commons.collections.ReferenceMap.get(Unknown Source)
at org.hibernate.util.SoftLimitMRUCache.get(SoftLimitMRUCache.java:51)
at org.hibernate.engine.query.QueryPlanCache.getNativeSQLQueryPlan()
at org.hibernate.impl.AbstractSessionImpl.getNativeSQLQueryPlan()
at org.hibernate.impl.AbstractSessionImpl.list()
at org.hibernate.impl.SQLQueryImpl.list(SQLQueryImpl.java:164)
at com.mogoko.struts.logic.user.LeaveMesManager.getCommentByShopId()
at com.mogoko.struts.action.shop.ShopIndexBaseInfoAction.execute()
... ...
在长达 5 分钟的时间里，一直在执行 org.apache.commons.collections.ReferenceMap.getEntry() 方法，说明这个方法一直没有结束。具体属于正常还是 Bug，需要结合源码进行判断。
导致死循环的代码属于代码的 Bug，这种类型的问题，重现比较难，但一旦重现，就比较容易解决。一般通过分析代码可以发现问题。导致死循环的原因大致有如下几个：
- HashMap 等线程不安全的容器，用在多线程读/写的场合，导致 HashMap 的方法调用形成死循环（可以参考2015-08-19 Hadoop升级过程出现异常问题casestudy）
- 多线程场合，对共享变量没有进行保护，导致数据混乱，从而使循环退出条件永远不满足，导致死循环的发生，比如：
- for, while 循环中的退出条件永远不满足导致的死循环
- 链表等数据结构首尾相连，导致遍历永远无法停止
- 其他编码错误
4.3 高消耗 CPU 代码的常用分析方法
借助操作系统提供的性能分析工具进行 CPU 消耗分析。上面介绍了针对死循环导致 CPU 遍告的分析方法，但是有些场景没有死循环也会导致 CPU 变高，我们需要借助系统提供的一些性能分析工具进行分析。在 Linux 中使用 top -p $pid 进行，步骤如下
- top -p $pid
- 输入 “H” 查看该进程所有线程的统计情况（CPU 等）如下所示（对于某些操作系统版本不支持 top 命令进行线程统计的
下面 PID 表示线程 ID，即 LWPID PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 25059 sankuai 20 0 9918m 55m 8264 S 0.0 0.2 0:00.00 java 25060 sankuai 20 0 9918m 55m 8264 S 0.0 0.2 0:00.04 java 25061 sankuai 20 0 9918m 55m 8264 S 0.0 0.2 0:00.00 java 25062 sankuai 20 0 9918m 55m 8264 S 0.0 0.2 0:00.00 java 25063 sankuai 20 0 9918m 55m 8264 S 0.0 0.2 0:00.00 java 25064 sankuai 20 0 9918m 55m 8264 S 0.0 0.2 0:00.00 java 25065 sankuai 20 0 9918m 55m 8264 S 0.0 0.2 0:00.00 java 25066 sankuai 20 0 9918m 55m 8264 S 0.0 0.2 0:00.00 java 25067 sankuai 20 0 9918m 55m 8264 S 0.0 0.2 0:00.00 java 25068 sankuai 20 0 9918m 55m 8264 S 0.0 0.2 0:00.00 java 25069 sankuai 20 0 9918m 55m 8264 S 0.0 0.2 0:01.19 java 25070 sankuai 20 0 9918m 55m 8264 S 0.0 0.2 0:00.00 java 25071 sankuai 20 0 9918m 55m 8264 S 0.0 0.2 0:00.00 java 25072 sankuai 20 0 9918m 55m 8264 S 0.0 0.2 0:00.00 java 25073 sankuai 20 0 9918m 55m 8264 S 0.0 0.2 0:00.00 java 25074 sankuai 20 0 9918m 55m 8264 S 0.0 0.2 0:00.00 java 25075 sankuai 20 0 9918m 55m 8264 S 0.0 0.2 0:00.00 java 25076 sankuai 20 0 9918m 55m 8264 S 0.0 0.2 0:24.31 java 25077 sankuai 20 0 9918m 55m 8264 S 0.0 0.2 0:01.00 java 25078 sankuai 20 0 9918m 55m 8264 S 0.0 0.2 0:01.05 java 25141 sankuai 20 0 9918m 55m 8264 S 0.0 0.2 0:00.00 java
具体导致问题的代码可能是：
- 纯 Java 代码导致的 CPU 过高
- Java 代码中调用的 JNI 代码导致的 CPU 过高
- 虚拟机自身的代码导致的 CPU 过高，比如 GC 的 bug 等
可以如下进行分析：
- top -p $pid 获取最消耗 CPU 的本地线程 ID
- jstack 打印出 Java 堆栈
- pstack 打印本地线程堆栈
- 在 Java 线程堆栈中查找 nid=<第一步中最消耗 CPU 时间的线程 id>
a. 如果找到的线程是纯 Java 代码，则该 Java 代码导致 CPU 过高
b. 如果找到的是 Native code，说明导致 CPU 过高的问题在 JNI 调用中
c. 如果找不到对于的线程，则有如下两种可能：i. JNI 调用中重新创建的线程来执行，在 Java 线程堆栈中不存在该线程的信息 ii. 虚拟机自身的代码导致的 CPU 过高，如堆内存不够导致的频繁 fullgc，或虚拟机的 bug 等
4.4 资源不足等导致的性能下降分析
大多数时候资源不足和性能瓶颈是统一类问题。
- 大量的线程停在同样的调用上下文上
- 资源数量配置太少，系统当前压力比较大，资源不足导致某些线程不能及时获得资源而等待
- 获得资源的线程把持资源时间太久（比如获取数据库连接），导致资源不足
- 设计不合理导致资源占用时间过久，如 SQL 语句设计不恰当，或者没有索引导致的数据库访问太慢等
- 资源用完后，某些异常情况下，没有关闭或回收，导致可用资源泄漏或减少
4.5 线程不退出导致的系统挂死分析
线程挂死，表现为每次打印线程堆栈，该线程都在同一个调用上下文，该类问题可以通过多次打印堆栈进行分析，导致线程挂死的原因有很多，如：
- 线程正在执行死循环代码
- 资源不足或资源泄漏，造成当前线程阻塞在锁对象上，长期得不到唤醒
- 如果当前程序和外部通信，当外部程序挂起无返回时，也会导致当前线程挂起
4.6 其他问题分析
- 多个锁导致的锁链分析 – 多个线程等待同一个锁
- 进行性能瓶颈分析
4.7 线程堆栈不能分析什么问题
线程堆栈定位问题，只能定位在当前线程上留下痕迹的问题，如线程死锁，线程挂死等，另外，定位由于锁的设计不恰当导致的性能问题，线程堆栈也是最有效的工具，因为性能问题时时刻刻反映在当前的线程统计状况上。但线程堆栈对于不留痕迹的问题，就无能为力了。例如：
- 线程为什么跑飞
- 并发的 Bug 导致的数据混乱
- 数据库梭镖的问题
- 在实际的系统中，系统的问题分为几种类型：
堆栈中能够表现出问题的，使用线程堆栈进行定位
- 无法在线程中留下痕迹的问题定位，需要依赖一个好的日志设计
- 非常隐蔽的问题，只能依赖于丰富的代码经验，如多线程导致的数据混乱以及幽灵代码等
正文到此结束
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